For obvious safety reasons, important items of equipment in an automotive vehicle are fitted with devices for detecting and indicating any operational anomaly.
The on-board generator, usually consisting of an alternator with rectified phase voltages, is one of these important items of equipment and every driver is familiar with the position of the red light marked “charge” on the dashboard of his car.
The widespread use of electronics on board vehicles, often as a replacement for electromechanical components, enables even finer detection of operating faults and of the circumstances of breakdowns, for purposes of either diagnostics, communication or protection of the equipment.
For example, the electronic voltage regulator described in patent application FR2642580 of the VALEO EQUIPEMENTS ELECTRIQUES MOTEUR company contains fault-detecting means which analyse the correspondence between various accessible parameters, in particular the correspondence between:                the voltage at the output of the alternator and the voltage at the terminals of the field winding;        the voltage at the output of the alternator and the voltage at the terminals of the phase inputs.        
The second case allows the following faults to be detected indirectly:                no rotation of the alternator (broken fan belt);        an open excitation circuit (open field winding, brush jammed in its housing, etc. . . . ).        
The VALEO EQUIPEMENTS ELECTRIQUES MOTEUR company in patent application FR2910639 described a similar method for finding a solution for continuing the excitation circuit applying if the magnetic circuit of the alternator comprises permanent magnets or exhibits strong residual magnetism.
Modern alternator regulators integrate a power switch, generally a transistor known as “high side”, which controls an excitation current flowing in an excitation winding of a rotor of the alternator.
The rotor creates a magnetic field, the value of which is adjusted by the excitation current, in order to maintain the DC voltage generated by the alternator at the desired value, after rectifying the phase voltages.
However the output of this power switch can be short-circuited, a type of failure of the excitation circuit which is not detected in the known methods pointed out above and which thus requires to be improved.